Is it permissible to acquire a car from the National Social Security Fund in Tunisia, with its price of ten thousand Tunisian dinars being paid in installments over five years, making the total amount paid fourteen thousand dinars?

If the Guarantee Fund purchases the car and it enters into its possession, and then sells it to you on an installment plan, there is no harm in that. This is a Murabaha sale for one who commands a purchase. However, if the Fund lends you its price and then recovers more from you, this is usury (riba), which is forbidden by Islamic law.